Prevent mixing of weights in a bone
You can delete what vertex group each individual vertex belongs to 

3D view Editor-->Edit mode-->(n) properties shelf-->vertex weights.



Imagine each vertex group = a muscle. 

The more vertex groups control a single vertex= the more muscles are connected to that vertex . 

So when you are trying to make the belly bigger with one muscle, the other muscles are still attached=you get uneven stretching.

And the muscle strength can only be changed manually in weight paint mode or by using the vertex weight modifier.